The following background information is provided to assist the reader in understanding the environment in which the invention will typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless specifically stated otherwise in this document.
Electrical load devices such as motors, solenoids, batteries, transformers and the like are employed in a variety of devices, machines and industrial processes. It is well known that the electrical load must change its state from OFF to ON in order to perform a designated function and change the state back to OFF after the function has been performed. Such changes in state are controlled by additional electrical devices forming a control circuit in combination with electrical load. Upon receiving a condition signal, the control circuit would enable the electrical load to change its state.
In certain safety applications such as light bars, contact mats or strips used for machine safety, locking devices employing solenoids or small motors or power operators for opening and closing doors in a transit vehicle, it is important to change the state of the electrical load only when predetermined conditions are verified within the control circuit. For example, to open the door of the transit vehicle it is required to verify that a door open signal is received in combination with the propulsion signal indicating that the vehicle is below a predetermined speed threshold. On the other hand, in order to activate the propulsion motors of a transit vehicle it is required to verify that sensors monitoring door position indicate that all doors have been closed.
In another aspect, it is required to change the state of the electrical load in a fail-safe manner and in a predetermined sequence eliminating an opportunity for component failures to enable such change in state. This especially applies to electronic devices that have various failure modes, sometimes difficult to characterize, with short-circuit and sensitivity to supply voltage transients mode being some of them.
Yet, in another aspect, it is desirable to monitor the change of state process in order to abort such process when failures occur and further to annunciate any abnormal conditions.